Design of advanced bainitic steels by optimisation of TTT diagrams and T-0 curves

Cementite is responsible of the limited application of conventional bainitic steels, however it has been proof that cementite precipitation during bainite formation can be suppressed by the judicious use of silicon in medium carbon steels. In this work, thermodynamic and kinetic models were used to design steels with an optimum bainitic microstructure consisting of a mixture of bainitic ferrite, carbon-enriched retained austenite and some martensite. Using these models, a set of seven carbide free bainitic steels with a 0.3 wt% carbon content were proposed for manufacturing. The work presented here is concerned with the microstructural and mechanical characterisation of the steels manufactured. Except for the steel with the highest content of alloying elements, all the grades present the same microstructure composed of carbide-free upper bainite and retained austenite after hot rolling and a two-steps cooling. Theirs tensile strengths range from 1 600 to 1 950 MPa while keeping a uniform elongation equal to 4 % and a total elongation over 10 %. Regarding toughness at room temperature, they match quenched and tempered martensitic steels.